The high-temperature solution process is a versatile way to make ethylene polymer resins that are useful for blow molding, injection molding, films, coatings, and other applications. A number of these are practiced commercially by Dow Chemical, Nova Chemicals, Equistar Chemicals, and others. One type of high-temperature solution process is conducted by making the same or different polymers in two parallel reactors in a first reaction zone (see, e.g., U.S. Pat. No. 5,236,998). Products from the two reactors are then combined in a second reaction zone where polymerization continues. Optionally, a third reaction zone is used to complete the process.
The high-temperature solution process is usually performed using one or more Ziegler-Natta catalysts, and the products are substantially linear ethylene homopolymers or copolymers. The products have short-chain branching when comonomers are included, but they normally have little or no long-chain branching. Although the available product slate from the solution process is rich, it could potentially expand if long-chain branching could be easily introduced into the polymers. For example, HDPE resins that have substantial long-chain branching should be valuable for shrink-film applications.
Some metallocenes can provide ethylene polymers having long-chain branching. These catalysts generally have relatively few active sites compared with Ziegler-Natta catalysts, so they are poisoned easily by trace impurities commonly present in monomer feedstocks. Additionally, many metallocenes have relatively low thermal stability. Consequently, better ways to introduce long-chain branching into polymers made in the high-temperature solution process are still needed.
Among Ziegler-Natta catalysts, those containing a combination of titanium, magnesium, and aluminum (e.g., see U.S. Pat. No. 4,499,198), particularly ones based on magnesium silylamides, offer some advantages over earlier varieties based on titanium and aluminum or vanadium and aluminum, including high activity and the ability to incorporate comonomers efficiently. Use of these advanced Ziegler-Natta catalysts to somehow influence the amount of long-chain branching in a polymer has not been described.
In sum, the industry would benefit from ways to modify the high-temperature solution process to provide ethylene polymers with a controllable amount of long-chain branching. Ideally, the process would avoid catalysts having a tendency to deactivate in the presence of trace feedstock impurities, such as many metallocenes. A valuable process would not require invention and development of new catalysts and could be practiced easily without the need to extensively modify plant equipment. An ideal process would allow polyolefin manufacturers to tailor polymer properties and processability by “dialing in” a desired level of long-chain branching.